Use Workflow to Handle Distributed Transactions | by dtm | Jul, 2022

Picture by Markus Spiske on Unsplash

On the earth of microservices, a transaction is now distributed to a number of companies which might be referred to as in a sequence to finish all the transaction.

With the arrival of microservice structure, we’re shedding the ACID nature of databases. Transactions could now span a number of microservices and, due to this fact, databases.

To deal with the issues of distributed transactions, the next record of approaches have been described:

  • Two-Part Commit / XA
  • Eventual Consistency and Compensation / SAGA
  • Attempt, Affirm, Cancel / TCC

In basic options, the builders ought to select one of many three patterns to deal with distributed transactions.

However on this article, we introduce a workflow sample in Underneath this sample, a combination of XA, SAGA and TCC may be utilized to completely different branches in a single distributed transactions, permitting customers to customise a lot of the contents of a distributed transaction, offering nice flexibility.

Within the Workflow mode of DTM, each HTTP and gRPC protocols can be utilized. The next is an instance of the gRPC protocol, which is split into the next steps:

  • Initialize the SDK
  • Register workflow
  • Execute workflow

First, you’ll want to Initialize the SDK’s workflow earlier than you should utilize it.

import ""// Initialize the workflow SDK with three parameters.
// the primary parameter, the dtm server handle
// the second parameter, the enterprise server handle
// The third parameter, grpcServer
// workflow must obtain the dtm server callback from the "enterprise server handle" + "grpcServer"
workflow.InitGrpc(dtmGrpcServer, busi.BusiGrpc, gsvr)

Then you’ll want to register workflow’s handler operate. The next is a saga workflow to do a cross-bank switch:

  • This registration operation must be executed after the enterprise service is began as a result of when the method crashes, dtm will name again to the enterprise server to proceed the unfinished process
  • The above code NewBranch will create a transaction department, one that may embrace a ahead motion and a callback on world transaction commit/rollback
  • OnRollback/OnCommit will register a callback on world transaction rollback/commit for the present transaction department. Within the above code, solely OnRollback is specified, so it’s in Saga mode
  • The busi.BusiCli within the above code wants so as to add a workflow interceptor which is able to mechanically document the outcomes of the rpc request to dtm as follows
conn1, err := grpc.Dial(busi.BusiGrpc, grpc.WithUnaryInterceptor(workflow.Interceptor), nossl)
busi.BusiCli = busi.NewBusiClient(conn1)

You’ll be able to, after all, add workflow.Interceptor to all gRPC shoppers, this middleware will solely deal with requests underneath wf.Context and wf.NewBranchContext()

  • When the workflow operate returns nil/ErrFailure, the worldwide transaction enters the Commit/Rollbasck part, calling the operations registered in OnCommit/OnRollback contained in the operate in reverse order

Lastly, the workflow is executed.

req := &busi.ReqGrpcAmount: 30
err = workflow.Execute(wfName, shortuuid.New(), dtmgimp.MustProtoMarshal(req))
  • When the results of Execute is nil/ErrFailure, the worldwide transaction has succeeded/been rolled again.
  • When the results of Execute is different values, the dtm server will subsequently name again this workflow process to retry

How does workflow guarantee knowledge consistency in distributed transactions? When a enterprise course of has a crash or different downside, the dtm server will discover that this workflow world transaction has timed out and never accomplished. Then the dtm server will use an exponential retreat algorithm and retry the workflow transaction. When the workflow retry request reaches the enterprise service, the SDK will question the progress of the worldwide transaction from the dtm server.

For the finished department, it can take the beforehand saved end result and return the department end result immediately via an interceptor comparable to gRPC/HTTP. Finally, the workflow will full efficiently.

Workflow capabilities must be idempotent, i.e., the primary name, or subsequent retries, ought to get the identical end result.

The core concept of the Saga sample, derived from this paper SAGAS, is that lengthy transactions are break up into brief transactions coordinated by the Saga transaction coordinator. If every brief transaction operation efficiently completes, then the worldwide transaction completes usually, and if a step fails, the compensating operations are invoked one by one in reverse order.

In Workflow mode, you may name the operate for the operation immediately within the operate after which write the compensation operation to OnRollback of the department, after which the compensation operation shall be referred to as mechanically, reaching the impact of Saga mode

The Tcc sample is derived from this paper, Life beyond Distributed Transactions: an Apostate’s Opinion. He divides a big transaction into a number of smaller transactions, every of which has three operations.

  • Attempt part: makes an attempt to execute, completes all enterprise checks, units apart sufficient enterprise assets
  • Affirm part: if the Attempt operation succeeds on all branches, it goes to the Affirm part, which executes the transaction with none enterprise checks, utilizing solely the enterprise assets put aside within the Attempt part
  • Cancel part: If one of many Attempt operations from all branches fails, we go to the Cancel part, which releases the enterprise assets reserved within the Attempt part.

For our state of affairs of an interbank switch from A to B, if SAGA is used and the steadiness is adjusted within the ahead operation and is adjusted reversely within the compensating operation, then the next state of affairs would happen.

  • A deducts the cash efficiently
  • A sees the steadiness lower and tells B
  • The switch of the quantity to B fails, and the entire transaction is rolled again
  • B by no means receives the funds

This causes nice misery to each As and Bs. This example can’t be averted in SAGA, however TCC can resolve it with the next design method:

  • Introduce a trading_balance discipline along with the steadiness discipline within the account
  • Attempt part to test if the account is frozen, test if the account balance-trading_balance is adequate, after which modify the trading_balance (i.e., the funds frozen for enterprise functions)
  • Affirm part, modify steadiness, modify trading_balance (i.e., unfrozen funds for the enterprise)
  • Cancel part, modify trading_balance (i.e., unfsrozen funds on the enterprise)

On this case, as soon as finish consumer A sees their sbalance deducted, then B should be capable to obtain the funds

In Workflow mode, you may name the Attempt operation immediately within the operate, then register the Affirm operation to OnCommit within the department and register the Cancel operation to OnRollback within the department, reaching the impact of the Tcc mode

XA is a specification for distributed transactions proposed by the X/Open group. The XA specification primarily defines the interface between a (world) Transaction Supervisor (TM) and a (native) Useful resource Supervisor (RM). Native databases comparable to MySQL play tse RM position within the XA.

XA is split into two phases.

  • Part 1 (put together): All individuals RM put together to execute the transaction and lock the required assets. When the individuals are prepared, they report back to TM that they’re prepared.
  • Part 2 (commit/rollback): When the transaction supervisor (TM) confirms that each one individuals (RMs) are prepared, it sends a commit command to all individuals.

At the moment, all main databases help XA transactions, together with MySQL, Oracle, sqlserver, and postgres

In Workflow mode, you may name NewBranch().DoXa within the workflow operate to open your XA transaction department.

In Workflow mode, Saga, Tcc, and XA, as described above, are all patterns of branching transactions. So you should utilize one sample for some branches and one other sample for others. The pliability provided by this combination of patterns permits for sub-patterns to be chosen in line with the traits of the department transaction, so the next is beneficial.

  • XA: If the enterprise has no row lock rivalry, and the worldwide transaction won’t final lengthy, XA can be utilized. This sample requires much less extra improvement and Commit/Rollback is finished mechanically by the database. For instance, this sample is appropriate for an order creation enterprise the place completely different orders lock completely different order rows and don’t have any impact on concurrency between one another. It isn’t appropriate for deducting stock as a result of orders involving the identical merchandise will all compete for the row lock of this merchandise, which is able to result in low concurrency.
  • Saga: A typical enterprise unsuitable for XA can use this mannequin. This mannequin has much less additional improvement than Tcc; it solely must develop ahead operation and compensation operation
  • Tcc: appropriate for prime consistency necessities, such because the switch described earlier, this sample has essentially the most extra improvement and requires the event of operations together with Attempt/Affirm/Cancel

Within the Workflow sample, when a crash happens, a retry is carried out so the person operations are required to help idempotency, i.e., the results of the primary name is similar as the subsequent strive, returning the identical end result.

In enterprise, the distinctive key of the database is normally used to attain idempotency, particularly insert ignore "unique-key". If the insert fails, it means this operation has been accomplished. This time immediately ignored to return. If the insert succeeds, it signifies that that is the primary operation, proceed with the following enterprise operations.

If your online business itself is idempotent, then you may function your online business immediately. If your online business doesn’t present idempotent performance, then dtm supplies a BranchBarrier helper class, based mostly on the above unique-key precept, which may simply assist builders implement idempotent operations for Mysql/Mongo/Redis.

Please observe that the next two are typical non-idempotent operations:

  • Timeout rollback: You probably have an operation in your online business which will take a very long time, and also you need your world transaction to roll again after ready for the timeout to return a failure. Then this isn’t an idempotent operation as a result of, within the excessive case of two processes calling the operation concurrently, one returns a timeout failure and the opposite a hit, leading to completely different outcomes.
  • Rollback after reaching the retry restrict: the evaluation course of is similar as above.

Workflow mode doesn’t help the above timeout rollback and retry restrict rollback for the time being. You probably have a related state of affairs, please ship us the precise state of affairs. We’ll actively take into account whether or not so as to add this sort of help.

Branching operations will return the next outcomes.

  • Success: the department operation returns HTTP-200/gRPC-nil
  • Enterprise failure: the department operation returns HTTP-409/gRPC-Aborted, no extra retries, and the worldwide transaction must be rolled again
  • In progress: department operation returns HTTP-425/gRPC-FailPrecondition. This end result signifies that the transaction is in progress usually and requires the dtm to retry not with the exponential retreat algorithm however with mounted interval retries
  • Unknown error: the department operation returns different outcomes, indicating an unknown error, and dtm will retry this workflow utilizing the exponential retreat algorithm

In case your current service has a unique end result than the above, then you may customise this a part of the end result with workflow.Choices.HTTPResp2DtmError/GRPCError2DtmError .

Saga’s Compensation operation and Tcc’s Affirm/Cancel operation aren’t allowed to return enterprise failures in line with the Saga and Tcc protocols, as a result of when within the second stage of the workflow, Commit/Rollback, is neither profitable nor allowed to retry, then the worldwide transaction can’t be accomplished, so please take care to keep away from this when designing.

Some enterprise eventualities the place you need to be notified of the completion of a transaction may be achieved by setting an OnFinish callback on the primary transaction department. By the point the callback known as, all enterprise operations have been carried out, and the worldwide transaction is considerably full. The callback operate can decide whether or not the worldwide transaction has lastly been dedicated or rolled again based mostly on the isCommit handed in.

One factor to notice is that when the OnFinish callback known as, the state of the transaction has not but been modified to the ultimate state on the dtm server. So if you happen to use a combination of transaction completion notifications and querying world transaction outcomes, the outcomes of the 2 will not be constant. It’s endorsed that customers use solely one in all these strategies moderately than a combination.

We introduce a workflow sample to help the combined utilization of Saga, XA, and Tcc. This sample additionally helps HTTP, gRPC, and native transactions.

We additionally illustrate the utilization of workflow in Golang with runnable examples.

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